Fixes #9089. On linux, pressing shift and tab together can potentially produce `ISO_Left_Tab`. This PR maps this key to "tab" with the shift modifier, similar to `SHIFT_TAB_KEY` in gpui::platform::mac::events. Note: The [default linux keymaps](https://github.com/zed-industries/zed/blob/main/assets/keymaps/default-linux.json) have shift-tab mapped to editor::TabPrev and ctrl-[ mapped to editor::Outdent. Both actions appear to have the same effect. Release Notes: - Support shift-tab on linux (#9089).